Implement transparency groups

Details

Description

The attached PDF uses transparency groups, blending and soft masks to create the rounded corners and shades behind images. It appears that these features are not implemented in PDFBox. An implementation proposal is attached in the TransparencyGroup.patch. The basic idea is to create a buffered image, draw the transparency group content onto it and then use the result to produce the soft mask or draw the image on the original g2d.

Note: I am not the (only) author of the proposed change. It was developed in our company few years ago in sources based on a 1.7.x version of PDFBox, mostly by a guy who already left. Over the years, merging of the work done in PDFBox main stream into our source base has become impossible due to many refactorings and other deep going changes done. Now we would like to go the opposite way - where possible - bring the changes and fixes we have done into PDFBox main stream and start to use it in our installations.